1. Field of the Invention
The present invention is generally directed to an optical fiber, and more particularly to a polarization mode dispersion (PMD) compensator for a single-mode optical fiber.
2. Technical Background
Polarization mode dispersion (PMD) in single-mode optical fibers is a recognized source of bit errors in high-speed optical communication systems. PMD may cause optical pulse broadening, or pulse deformation in general, and as such, limit the bit rate that can be achieved with a given optical communication system that does not compensate for the PMD. As pulses broaden, eventually the individual bits are no longer distinguishable and the communication system ceases to properly function.
PMD in single-mode optical fibers has been explained through a model which divides a given light pulse into a signal with two principal states of polarization (PSP). The pulse broadening of a transferred optical signal, induced by first-order PMD, is caused by the propagation time difference between the input pulses projected onto each of the two orthogonal polarization axes (i.e., the PSP) in a single span of single-mode optical fiber or onto the PSP of interconnected single-mode optical fibers. The propagation time difference, known as differential group delay (DGD), is usually expressed in units of picoseconds per square root kilometer for single spans of single-mode optical fiber and for interconnected single-mode optical fibers.
As is well known to one of ordinary skill in the art, first-order PMD is typically caused by a characteristic of optical fibers known as birefringence. Birefringence occurs when an optical fiber has different indices of refraction, with respect to a set of axes defined within the fiber. For example, when light travels along the length of the fiber (e.g., along the Z axis of a Cartesian coordinate system), the two principal states of polarization are orthogonal to each other and to the length of the fiber (e.g., projected along the X and Y axes). The indices of refraction measured with respect to the X and Y axes typically exhibit slight differences due to the geometrical properties of the fiber and/or environmental effects.
Many geometrical deviations of an optical fiber can be attributed to the optical fiber manufacturing process, which typically yields fibers that are not perfectly round. Environmental effects, such as applied stress, can alter the index of refraction in the region of the stress and cause the birefringence of the fiber to change. The change in birefringence with the application of stress has been extensively studied in the field of stress optics. When the amount of stress, the type of stress and the stress properties of a given optical fiber are known, the amount of change in birefringence can generally be accurately calculated. The fiber axis with the higher index of refraction is known as the xe2x80x98slow axisxe2x80x99. The fiber axis with the lower index of refraction is known as the xe2x80x98fast axisxe2x80x99. The terms xe2x80x98fast axisxe2x80x99 and xe2x80x98slow axisxe2x80x99 refer to the relative speed of light propagation along the orthogonal axes. As is well known to one of ordinary skill in the art, a change in birefringence, as a function of applied stress, is typically greatest when the stress is applied transverse to either the fast or slow axis of an optical fiber.
Several techniques for changing the birefringence of an optical fiber are known. For example, the birefringence of an optical fiber can be changed by applying an axial strain (pulling) or applying a transverse stress to the fiber, with a piezoelectric actuator or an electromagnetic fiber squeezer. However, pulling on a fiber is subject to mechanical limitations that include breakage and coating delamination and therefore is somewhat less desirable than applying a transverse stress. As compared to a piezoelectric actuator, a typical electromagnetic fiber squeezer is slow, uses more power and is noisier (i.e., is a noise source in the electromagnetic spectrum).
A variety of arrangements have been proposed as potential PMD compensators for optical fibers. The different PMD compensators can generally be classified according to how polarization transformation and compensation is accomplished. For example, polarization transformation has been achieved by mechanical rotating elements, liquid crystals and fiber squeezers. As is well known to one of ordinary skill in the art, squeezing an optical fiber induces a stress birefringence, which can be utilized to control polarization. Current PMD compensator designs utilize optical or opto-electronic birefringent elements that permit the delay of one polarization state with respect to the other. While certain experimental PMD compensators have been demonstrated, no practical PMD compensators, which adequately compensate for first-order PMD in optical fibers, are commercially available.
As such, a practical device which compensates for first-order polarization mode dispersion in a single-mode optical fiber, is desirable.
The present invention is directed to a polarization mode dispersion compensator for correcting polarization mode dispersion in an optical signal having a fast polarization mode component, a slow polarization mode component and a time differential between the components. The compensator includes a phase shifter and a variable delay section. An input of the phase shifter is coupled to an optical device that provides an optical signal that exhibits polarization mode dispersion. The phase shifter functions to rotate the optical signal principal states of polarization to a desired orientation. The variable delay section includes an input, an output and at least one optical fiber delay line. The input of the variable delay section is coupled to the output of the phase shifter and the desired orientation of the optical signal principal states of polarization are substantially rotated to be in alignment with one of a fast axis and a slow axis of each of the one or more fiber delay lines. In this manner, the variable delay section functions to delay the principal states of polarization of the optical signal with respect to one another as a function of whether the principal states of polarization traverse said one of a fast and a slow axis of a given optical fiber delay line, thus reducing the time differential between them.
The invention also includes an inventive coating for optical fiber that is included in the polarization mode dispersion compensator. The inventive coating is preferably a radiation cured coating which is applied to at least a segment of an outer surface of the fiber. Preferably, the inventive coating composition is selected so that in response to a preload comprising the application of a stress of about 80 MPa to the coating at about 80xc2x0 C. and after a stress-relaxation period of at least about 1 hour, at about 80xc2x0 C., a residual stress exhibited by the coating comprises at least about 60 MPa.
Advantages of the inventive coating include the ability to substantially transmit a lateral stress applied to the coating to a glass fiber, the coating is substantially elastic, and will substantially maintain the transmission of the stress to a fiber over time. Preferably, the stress transmitted will not undergo substantial decay as a function of time. The advantages of the coating also include that the coating has the ability to transmit a transverse stress to the fiber to controllably change the birefringence of the fiber. The advantages further include that the coating has the ability to maintain the stress after a creep period. In practice the xe2x80x9ccreep periodxe2x80x9d is the time during which the actual build-up of the device takes place, where the static load is manually applied to the coated fiber via the squeezer plates and the set-screw to achieve the desired optical output.
In an optional embodiment, the inventive coating comprises about 0-90 weight percent of an oligomeric component and about 5-97 weight percent of a monomeric component. Preferably, the Young""s modulus of the aforementioned embodiment of the coating comprises at least about 100 MPa.
A PMD compensator, according to the present invention, is relatively easy to manufacture and provides a low loss, low cost and highly reliable practical solution to PMD.
Additional features and advantages of the invention will be set forth in the detailed description which follows and will be apparent to those skilled in the art from the description or recognized by practicing the invention as described in the description which follows together with the claims and appended drawings.
It is to be understood that the foregoing description is exemplary of the invention only and is intended to provide an overview for the understanding of the nature and character of the invention as it is defined by the claims. The accompanying drawings are included to provide a further understanding of the invention and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments of the invention which, together with their description serve to explain the principals and operation of the invention.